The present invention relates, in general, to a screw for a plastics processing machine. The present invention further relates to a method of regenerating a screw for a plastics processing machine.
Regeneration or rehabilitation of screws, such as extruder screws, that are no longer useful as a result of wear is normally realized by grinding the worn-out screw flights to a uniform reference diameter and subsequently building up a new coating which corresponds to the desired size or the dimensions of a new screw and has the required wearing protective layer upon the crest of the screw flights.
In general, an extruder screw becomes useless and must be regenerated, when wear leads to a decrease in diameter of about 2 mm. According to a conventional regeneration process, the screw flights are at first ground to a uniform reference diameter and coated during a subsequent hard-facing step through welding with a buffer layer which essentially replaces again the flight regions reduced through wear and erosion.
The surface of the weld-on buffer layer is polished and subsequently lined with a wearing protective layer by means of a thermal spraying process. The melt applied during the hard-facing step exposes the base material of the screw to great thermal stress, which may result in structural changes that impair the adhesion capability of the layer and in crack formation. Furthermore, there is a risk of distortion of the screw, thereby adversely affecting the rotational accuracy. The required alignment process involves a further risk of formation of cracks. Moreover, screws with chromium-plated screw threads suffer the problem of separation of the chromium layer from the screw flank in the area of melt application.
Swiss Pat. No. CH 669212 discloses a method of making an extruder screw in which the screw root as well as the screw flights and a wear-resistant material layer are built-up solely from individual metal layers through thermal coating technique. In this process, the geometric buildup of the screw flights is in the foreground, whereby no measures are disclosed as to how in a base screw body made of a uniform base material and including the screw flights, the worn-off screw can be so regenerated again that a wear-reducing coating can be connected to the base material, without risk of detachment.
It would therefore be desirable and advantageous to provide an improved screw for a plastics processing machine and an improved method of regenerating a worn screw, to obviate prior art shortcomings and to provide a regenerated screw with a long service life while being capable to absorb high loads.
According to one aspect of the present invention, a screw for a plastics processing machine, includes a screw body made of a base material, which is no longer useful as a consequence of wear, with the screw body including a screw flight having a crest; a buffer layer made of magnetically conductive metallic material and applied onto the screw crest of the screw body through a thermal spraying process; and a wear-reducing coating made of molybdenum or a molybdenum-containing alloy and applied onto the buffer layer through a thermal spraying process.
The buffer layer may be a single-ply or multi-ply buffer layer and may be made of chromium steel with a content of Cr of 10 to 16 wt. %, preferably 13 wt %. Te buffer layer may have a thickness in the range from 0.1 to 1.2 mm. The coating may contain at least at 30 wt % of molybdenum, or may be made almost entirely of molybdenum. The thickness of the coating may be in the range of 0.3 to 0.5 mm, preferably 0.4 mm.
According to another aspect of the present invention, a method of regenerating a worn extruder screw, includes the steps of abrading the crest of screw flights of a screw body to a uniform diameter; depositing a buffer layer of magnetically conductive metallic material onto the crest at a thickness to compensate for a diameter reduction of the screw body as a consequence of wear and abrasion to a uniform diameter; and applying a wear-reducing coating of molybdenum or a molybdenum-containing alloy onto the buffer layer through a thermal spraying process.
According to another feature of the present invention, the surface of the abraded screw body is suitably roughened through a blasting process.
According to another feature of the present invention, the thermal spraying process may be a wire flame spraying process.